Electric furnace for production of magnesium



Patented July 25, 1950 ELECTRIC FURNACE FOR PRODUCTION OF MAGNESIUM Valdemar James Melsted, Listerhll, Ala., assignor to Aluminium'Labox-atories Limited, Montreal,

Quebec, Canada.,

a corporation of Canada Application July 29, 1948, Serial No. 41,276 In Canada August 9, 1947 This invention relates to apparatus for the production of magnesium by thermal reduction of magnesium containing materials.

It is known to carry out such a reduction at reduced pressure in a retort in the presence of a reducing agent such as ferro-silicon, aluminium and the like, magnesium vapor being evolved when a temperature above 1000o C. is attained and this being drawn ofi and recovered in a suitable condenser, and it is usual t form the magnesium containing material and reducing agent into briquettes. 'I'he prior Work done has revealed two principal difficulties, namely, that of heating a large mass of such briquettes or of granules in a uniform manner, and that of removing the gaseous reaction products from the reaction zone at a rate suiliciently great to prevent their building up to a point such as to seriously decrease the rate of the desired reaction (by the mass action effect). Attempts have been made to minimize these diiiiculties but the apparatus proposed has been subject to disadvantages. The most satisfactory prior apparatus of which the applicant is aware deals with the second diiculty by laying the retort on its side so that the gaseous reaction products may reach an unfilled region above the charge and then be free to flow longitudinally to the condenser. It has, however, only been possible to ensure essentially uniform heat distribution by using retorts of a relatively small diameter, so that it has been necessary in practice to use a large number of retorts, each handling only a small batch.

According to the present invention there is provided apparatus which provides for the rapid removal of the gaseous reaction products from the reaction zone and at the same time ensures uniform heat distribution without limiting the size of the retort. Thus there is provided according to the invention a single means for heating the charge and also for carrying away the gaseous reaction products comprising a plurality of tubular resistance heating elements distributed throughout the charge, each element having a pluralityof perforations in the wall thereof, and 5 means connecting the elements to a source of electric current. The tubular elements distributed throughout the charge are readily spaced a distance apart such that the gaseous reaction products have in no case to travel far in order to escape from the charge through the perforations in a tubular element, and the size of the retort may be increased as desired on increasing the number of heating elements.

The tubular elements, which may be cylindrical Il or of any desired shape in cross section, are preferably upright and preferably open at both n ends.

- 'Ihe invention will now be described with reference to the accompanying drawings, in which Figure 1 shows an elevation partly in section of a retort provided with a plurality of tubular resistance heating elements according to the invention;

Fig. 2 is a section on the line 2--2 of Fig. l;

Fig. 3 is a detail illustrating a cylindrical heating element, and

Fig. 4 is a similar detail illustrating a tubular resistance heating element which is square in cross section.

'I'he retort IU may be of any desired form having a refractory lining I I and a vacuum tight metallic shell I2 of sullicient strength to resist collapse under the required vacuum. The top wall of the retort is provided with an opening I3 for the introduction of the charge and this in turn is provided with a suitable hatch (not shown). The lower end of the retort is funnelshaped and provided with an outlet I4 for the spent charge having suitable closure means (not shown). The retort may be supported on legs I 5.

A plurality of tubular resistance heating elements I6 are positioned Within the retort, the number of the elements depending upon the diameter of the retort. Each resistance element is provided with a plurality or" perforations I'I and the group of resistance elements is held together as a unit by electrical conductors I8-23, 23. The unit may be supported on the wall of the retort by insulating connectors 24. The resistance elements are connected to an external source of electric current (shown in diagram at 24a) by leads 25 and 26 which pass through the wall's of the retort and are sealed by means of gas-tight seals 2T, insulating bushings 28 being provided. The leads 25 and 26 and the electrical connectors Ill-23, 23', are conveniently connected to the tubular resistance heating elements by welding, but may be connected by bolting, clamping or other suitable means.

In the drawings the heating elements are shown connected in parallel but may be oonnected in series orseries parallel.

A pipe 29 passed through the wall of the retort and sealed by means of a gas-tight seal 30 leads to a suitable condenser for the magnesium vapors which, having passed upward through the tubular heating elements gather at the top of the retort above the charge which may be lled up above the level of the top of the 3 elements but is preferably kept below that level. A vacuum pump, or other suitable means of exhausting the apparatus, is connected to the outlet of the condenser.

The heating elements are shown projecting above the level of the charge here indicated at 32, but that is not essential nor need the tubes be open at either end. It is, however, preferred that they be open at both ends, and that the top end project above the charge as illustrated. The elements may be made of any suitable material or combination or assembly of materials and are preferably constructed of a metal or alloy which exhibits high electrical resistance and good strength at elevated temperatures. Certain alloys of iron, chromium and nickel have proved very useful, the so-called stainless steels being characterized by electrical resistance and other properties suitable for this` purpose.

The number and location of existing elements varies according to the size and shape of the cavity of the retort, the heating elements being of such a nature that they may be readily distributed throughout the retort in a manner such as to ensure uniformity of temperature throughout the charge and a rapid removal of the gaseous reaction products. The distribution and size of the perforations in the walls of the heating elements depends upon the particular reaction mixture. The perforations must be small enough to prevent pieces of the small reactioncharge from entering the interior of the heating elements and it is desirable to make the perforations slightly smaller than the smallest lump in the granular charge. While they may not be larger than such lumps, however, they may be considerably smaller.

It is to be understood that the invention is not limited to the specific apparatus herein shown and described but may be embodied in other forms without departure from its spirit.

I claim:

In furnace apparatus for thermal reduction of magnesium containing material, in combination, a retort comprising a shell which encloses a chamber that is adapted to receive a charge of divided, solid material, means extending through an upper part of said shell and opening into the upper part of said chamber, for withdrawal of gas from the chamber, a multiplicity of tubular resistance heating elements disposed side by side in said chamber, each of said elements comprising a tubular member having upper and lower ends and extending substantially vertically between its ends and having a plurality of perforations in its wall through which gaseous reaction products may pass, each element comprising structure electrically energizable by passage of electricity from one end to the other of the element for conversion of said electricity into 4 heat by resistance heating effect, said tubular elements being distributed in mutually spaced relation, substantially parallel to each other, throughout said chamber, a framework mounted in the aforesaid shell and extending laterally across the chamber below the top thereof, said framework including a plurality of electrically conducting elements connected to the upper ends of the tubular elements for supporting the latter and for conduction of electricity between the tubular elements and the conducting elements, a second framework mounted in the aforesaid shell and extending laterally across the chamber above the bottom thereof, said second framework including a plurality of electrically conducting elements connected to the lower ends of the tubular elements for supporting the latter and for conduction of electricity between the tubular elements and said last-mentioned conducting elements, said rst and second-mentioned conducting elements being cooperatively arranged to eiect ow of electricity through each of said tubular elements between its ends and thereby constituting means for supply of electric ourrent to said tubular elements, and means extending through the shell of the retort for connecting said last-mentioned means to a source of electric current.

VALDEMAR JAMES MELSTED.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 614,927 Burton Nov. 29, 1898 614,930 Burton Nov. 29, 1898 '124,705 Johnson et al. Apr. 7, 1903 920,473 Johnson May 4, 1909 957,058 Hixon May 3, 1910 1,054,380` Allen Feb. 25, 1913 1,103,060 Landis July 14, 1914 1,357,901 Reed Nov. 2, 1920 1,406,851 Hadaway, Jr. Feb. 14, 1922 1,640,735 Soderberg Aug. 30, 1927 1,680,630 Okell Aug. 14, 1928 2,244,267 Slayter et al June 3, 1941 2,386,189 Bagley Oct. 9, 1945 2,415,494 Holden Feb. 11, 1947 FOREIGN PATENTS Number Country Date 150,853 Switzerland Feb. l, 1932 OTHER REFERENCES Fowle: Standard Handbook for Electrical Engineers, McGraw-Hill Book Company, 6th edition; 1933; page 1956, paragraph 23. 

